1. Field of the Invention
This invention relates in general to a direct access storage device (DASD) of the type utilizing magnetoresistive read sensors for reading signals recorded in a magnetic medium and, more particularly, it relates to a DASD having a novel magnetic tunnel junction (MTJ) sensor for minimizing the effect of thermal asperities.
2. Description of Related Art
Computers often include auxiliary memory storage devices having media on which data can be written and from which data can be read for later use. A direct access storage device (disk drive) incorporating rotating magnetic disks is commonly used for storing data in magnetic form on the disk surfaces. Data is recorded on concentric, radially spaced tracks on the disk surfaces. Magnetic heads including read sensors are then used to read data from the tracks on the disk surfaces.
In high capacity disk drives, magnetoresistive read sensors, commonly referred to as MR heads, are the prevailing read sensor because of their capability to read data from a surface of a disk at greater track and linear densities than thin film inductive heads. An MR sensor detects a magnetic field through the change in the resistance of its MR sensing layer (also referred to as an "MR element") as a function of the strength and direction of the magnetic flux being sensed by the MR layer.
The conventional MR sensor operates on the basis of the anisotropic magnetoresistive (AMR) effect in which an MR element resistance varies as the square of the cosine of the angle between the magnetization in the MR element and the direction of sense current flowing through the MR element. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in the direction of magnetization in the MR element, which in turn causes a change in resistance in the MR element and a corresponding change in the sensed current or voltage.
Another type of MR sensor is the giant magnetoresistance (GMR) sensor manifesting the GMR effect. In GMR sensors, the resistance of the MR sensing layer varies as a function of the spin-dependent transmission of the conduction electrons between magnetic layers separated by a non-magnetic layer (spacer) and the accompanying spin-dependent scattering which takes place at the interface of the magnetic and non-magnetic layers and within the magnetic layers.
GMR sensors using only two layers of ferromagnetic material (e.g., Ni--Fe or Co or Ni--Fe--Co of Ni--Fe/Co) separated by a layer of non-magnetic material (e.g., copper) are generally referred to as spin valve (SV) sensors manifesting the SV effect.
FIG. 1 shows a prior art SV sensor 100 comprising end regions 104 and 106 separated by a central region 102. A first ferromagnetic layer, referred to as a pinned layer 120, has its magnetization typically fixed (pinned) by exchange coupling with an antiferromagnetic (AFM) layer 125. The magnetization of a second ferromagnetic layer, referred to as a free layer 110, is not fixed and is free to rotate in response to the magnetic field from the recorded magnetic medium (the signal field). The free layer 110 is separated from the pinned layer 120 by a non-magnetic, electrically conducting spacer layer 115. Hard bias layers 130 and 135 formed in the end regions 104 and 106, respectively, provide longitudinal bias for the free layer 110. Leads 140 and 145 formed on hard bias layers 130 and 135, respectively, provide electrical connections for sensing the resistance of the SV sensor 100. IBM's U.S. Pat. No. 5,206,590 granted to Dieny et al., incorporated herein by reference, discloses a GMR sensor operating on the basis of the SV effect.
Another type of magnetic device currently under development is a magnetic tunnel junction (MTJ) device. The MTJ device has potential applications as a memory cell and as a magnetic field sensor. The MTJ device comprises two ferromagnetic layers separated by a thin, electrically insulating, tunnel barrier layer. The tunnel barrier layer is sufficiently thin that quantum-mechanical tunneling of charge carriers occurs between the ferromagnetic layers. The tunneling process is electron spin dependent, which means that the tunneling current across the junction depends on the spin-dependent electronic properties of the ferromagnetic materials and is a function of the relative orientation of the magnetic moments, or magnetization directions, of the two ferromagnetic layers. In the MTJ sensor, one ferromagnetic layer has its magnetic moment fixed, or pinned, and the other ferromagnetic layer has its magnetic moment free to rotate in response to an external magnetic field from the recording medium (the signal field). When an electric potential is applied between the two ferromagnetic layers, the sensor resistance is a function of the tunneling current across the insulating layer between the ferromagnetic layers. Since the tunneling current that flows perpendicularly through the tunnel barrier layer depends on the relative magnetization directions of the two ferromagnetic layers, recorded data can be read from a magnetic medium because the signal field causes a change of direction of magnetization of the free layer, which in turn causes a change in resistance of the MTJ sensor and a corresponding change in the sensed current or voltage. IBM's U.S. Pat. No. 5,650,958 granted to Gallagher et al., incorporated in its entirety herein by reference, discloses an MTJ sensor operating on the basis of the magnetic tunnel junction effect.
FIG. 2 shows a prior art MTJ sensor 200 comprising a first electrode 204, a second electrode 202, and a tunnel barrier 215. The first electrode 204 comprises a pinned layer (pinned ferromagnetic layer) 220, an antiferromagnetic (AFM) layer 230, and a seed layer 240. The magnetization of the pinned layer 220 is fixed through exchange coupling with the AFM layer 230. The second electrode 202 comprises a free layer (free ferromagnetic layer) 210 and a cap layer 205. The free layer 210 is separated from the pinned layer 220 by a non-magnetic, electrically insulating tunnel barrier layer 215. In the absence of an external magnetic field, the free layer 210 has its magnetization oriented in the direction shown by arrow 212, that is, generally perpendicular to the magnetization direction of the pinned layer 220 shown by arrow 222 (tail of the arrow that is pointing into the plane of the paper). A first lead 260 and a second lead 265 formed in contact with first electrode 204 and second electrode 202, respectively, provide electrical connections for the flow of sensing current I.sub.s from a current source 270 to the MTJ sensor 200. A signal detector 280, typically including a recording channel such as a partial-response maximum-likelihood (PRML) channel, connected to the first and second leads 260 and 265 senses the change in resistance due to changes induced in the free layer 210 by the external magnetic field.
As mentioned earlier, an MR sensor exhibits a change in resistance when in the presence of a changing magnetic field. This resistance change is transformed into a voltage signal by passing a constant sense current through the MR element. The value of the DC voltage for a given MR sensor is the product of the constant sense current and the total resistance between the MR sensor leads. Since the change in the resistance is the principal upon which the MR sensor operates, the change in resistance can substantially effect the performance of the MR sensor and the disk drive incorporating the MR sensor.
A phenomena, known as a thermal asperity (TA), can suddenly increase the MR sensor temperature by more than 100 degrees C. The cause of this sudden temperature rise is a collision or near collision of the MR sensor with a protrusion on the disk surface while reading information from a track. The collision causes the DC base voltage of the MR sensor to shift substantially thus making the information unreadable.
FIG. 3 is a graph illustrating the DC base (bias) voltage 310, the thermal asperity voltage 320, which is the shift and decay in the base DC voltage 310, the data signal 335 read back from the disk in the absence of the thermal asperity 320, and the data signal 340 read back from the disk in the presence of the thermal asperity 320. Note that the thermal asperity 320 comprises a sudden shift 325 in the DC base voltage followed by an exponential decay 330 in the DC base voltage. The exponential decay 330 in the DC base voltage continues until the DC base voltage 310 is reached. It should be noted that the sudden shift 325 in the DC base voltage could be several times larger than the data signal 335 causing the electrical circuitry connected directly or indirectly to the MR sensor to saturate leading to the loss of the data. The loss of the data, depending on the size of the thermal asperity 320, could very easily be several bytes long, each byte being eight bits long.
Known arrangements in disk drives for minimizing the effect of thermal asperity on the read data utilize either a separate asperity reduction circuit (ARC) module which is costly or a complicated data channel (such as modified partial-response maximum likelihood channel) having a normal operating mode and an asperity recovery mode.
MR sensors and spin valve sensors with thermal asperity reduction circuitry to compensate for the effect of thermal asperities are described in IBM's U.S. Pat. No. 5,793,576 to Gill and in IBM's U.S. Pat. No. 5,793,207 to Gill, respectively, the contents of which are incorporated herein by reference. These applications describe sensors having four leads, two leads for providing sense current to the MR or SV sensor and two leads for providing current to an asperity compensation layer. The voltages developed across the MR or SV element (voltages due to the presence of thermal asperities and voltages due to the presence of data fields) and the asperity compensation layer (voltages due to the presence of thermal asperities) are applied to the inputs of a differential amplifier for substantial elimination of the thermal asperity signal.
Thermal asperities present similar problems for MTJ sensors as for MR and SV sensors. The MTJ sensor resistance changes as the spin dependent tunneling current through the tunnel barrier layer in the sensor changes due to the magnetic field of the data recorded on the disk. The base resistance of the MTJ sensor due to a constant bias voltage applied across the tunnel barrier layer decreases when a thermal asperity causes an increase of the temperature of the tunnel barrier layer. The resulting exponentially decaying thermal asperity signal interferes with detection of the superimposed data signal.
Therefore, there is a need for an invention that minimizes the effect of thermal asperities for MTJ sensors without utilizing a complicated recording channel or a separate ARC module.